Repeater link system



May 30, 1950 E. M. DELORAINE REPEATER LINK SYSTEM 11 Sheets-Sheet l Filed April 20, 1944 May 30, 1950 M. DELORAINE REPEATER LINK SYSTEM Filed April 20,1944

l1 Sheets-Shreve?I 2 A TTOIPME'Y May 30, 1950. E. M. DELoRAlNE 2,509,218

REPEATER LINK SYSTEM IN V EN TOR. UMC/V0 M DEZO/PH/NE TTRN ay 30, i950 E. M. DELORAINE 2,509,23

REPEATER LNK SYSTEM Filed April 20, 1944 11 Sheets-Sheet 4 ATTORNEY Ma 30, 1950 E. M. DELORAINE 2,509,218

REPEATER LINK SYSTEM Filed April 20, 1944 l1 Sheets-Sheet 5 2a 4a Gamma Na 2a 4a la Ja .5a 7a sa la 1a salia l H|i|||1| ATT/VEY 30, 1950 E. M. DELoRAlNE 2,509,218

REPEATER LINK SYSTEM Filed April 20,' 1944 l1 Sheets-Sheet 6 30, 1950 E. M. DELORAINE 2,5092@ REPEATER LINK SYSTEM Filed April 2o, 1944 11. sheets-sheet 7 INVENTOR. ED/v/o/vo M. @a0/Wwf May 30, 1950 Filed April 20. 1944 E. M. DELORAINE REPEATER LINK SYSTEM Qgg. j

11 Sheets-Sheet 8 A TT HIVE Y May 30, 1950 E. M` DELORAINE REPEATER LNK SYSTEM Filed April 20, 1944 Sheets-Sheet 9 1N VEN TOR.

Blf/37%@ REPEATER LINK SYSTEM Filed April 2o,V 1944 1i sheets-sheet 1o /V i Vg" 1/ BY @Q7/? May 3G, w50 E. M. DELQRAINE 25mm@ REPEATER LINK SYSTEM Patented May 30, 1950 REPEATER LINK SYSTEM Edmond M. Deloraine, New York, N. Y., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application April 20, 1944, Serial No. 531,851

' This invention relates to communication systems and more particularly to a radio multichannel communicating system particularly adapted for association with predetermined routes for aircraft, ships, railroad trains, automobiles and other vehicles with which communication may be desired.

One'of the objects of this invention is to pro- `vide a radio communicating system and method A4capable of communication with vehicles travelling Agenerally along a chain of relay stations forming the line of communication between given v terminals of the system.

Another object is to provide a radio multichannel communicating system and method .utilizing a chain of relay stations of such character that communication may readily be had Ibetween the terminals or other points in the Asystem and one or more vehicles moving genierally along the line of relay stations, either together on the same channel or selectively on ldiierent channels, or communication may be had between any two of such vehicles, the path fof transmission therebetween including at least partof the relay stations of the system.

Another object is to provide a multi-channel fcommunication systeml and method utilizing a fchai-n -of relay stations disposed along the path of vehicular travel by which channels can be selectively transmitted to selected relay stations where they are transmitted at broad angle radiation for reception by a nearby vehicle, and further to thereafter suppress the pulses in the relay equipment and insert reply channel pulses in the positions left by the suppressed pulses.

Another object is to provide a relay method and means for multi-channel communicating systems capable of repeating with directivity a plurality of channels of communication and for broadcasting at least certain of the channels for local reception and/or for suppressing certain outbranching channels and/or for inserting in- -branching channels of communication into the `communicating system.

- Still another object of the invention is to provide a channel inserting circuit for multi-chan- `nel communicating systems for inserting one or :more series of pulses, each series representing a lchannel of communication, in predetermined in- .:tervals in a train of multi-channel pulses.

A further object of the invention is to provide a translator circuit for multiplex signalling systems capable of segregating the signals received simultaneously from different sources and of translating the different signals into a characteristic (Cl. Z50-9) 2 modulation of pulses representing different channels of communication.

A still further object of the invention is to provide a transmitter-receiver unit -for multichannel communicating systems lcapable of receiving designated channels of communication and for selectively transmitting in other channels of the system. I

While this invention is applicable for communication with various types of vehicles, it is particularly yadaptable for use in connection with aircraft. The relay stations of the system are located at spaced points along a predetermined range or airway provided for guiding aircraft. The relay stations are preferably directive in character and are located apart in optical distances of 20 or 30 miles more or less, whereby low-powered transmission in the order of 1 to 2 watts in the ultra high frequency range is possible. Each relay station includes circuits for broadcasting at least certain `of the channels, either simultaneously or selectively, for reception by nearby aircraft in flight along the airway, and means for receiving messages transmitted from the aircraft and for inserting the messages in appropriate channels in the system. Each aircraft has a simple lightweight transmitter-receiver unit for reception of the channels broadcast from the relay stations and for transmitting messages for insertion in selected channels of the system. The channels of communication are :preferably controlled yat a given terminal where,

by means of an order channel, aircraft and the A next succeeding terminal station of the system can be called in for communication on selected channels. The aircraft, of course, are also provided with an `order channel over which they may request a channel for communication and over which navigation data may be supplied `from time to time.

It will be readily apparent that the communication system of this invention is readily applicable for numerous -uses in addition to the communicating feature, for example, the system may be used for controlling the radio range required for guiding aircraft, for registering at a terminal the location of aircraft in flight, controlling the landing and traffic of aircraft, transmitting navigation data and control signals for aircraft in flight, etc. The instant application, however, is concerned mainly with the communicating features whereby communication may be had between aircraft and terminals or other points in the system.

For a more complete understanding of the invention, reference may be had to the following detailed description to be read in connection with the accompanying drawings, in which:

Fig. 1 is a diagrammatical view in perspective showing two terminals of the multi-channel communicating system together with a chain of relay stations therebetween and showing aircraft in night along the line of relay stations;

Fig. 1A is a graphical illustration of a train of pulses which represent the different channels transmitted from the west base, and also the manner by which the channel pulses Iare monitored at receiver equipment;

Fig. 2 is a block diagram of. the west base terminal of the system;

Fig. 3 is a block diagram of the tower equipment for those relay stations at which channels are to be broadcast and messages inserted into the train or trains of channel pulses;

3Ais a diagrammatical showing ofvtwoA different angle directive radiation antennas that may be employed in the system; Y

Fig. 4 is a block diagram of the transmitterreceiver unit for aircraft and other vehicles;

Fig. 5 is a graphical illustration of a train of pulses representing dierent channels as received at a given relay station and also a graphical indication of how pulses of additional channels are inserted into the train of channel pulses;

Fig. 6 is a wiring diagram of a time modulator of a character that vmay be used in the system;

Fig. 7 is a wiring diagram of a selector and demodulator equipment that may be used in the system; Y

Fig. 8 is a circuit diagram of a pulse width selectory that may be used in the translator portion of the'tower equipment of Fig. 3;

9 is a graphical illustration used for explaining the operation of the pulse width selector of Fig. 8;

Fig. 10 is a block diagram of a modified form of equipment that may be used at relaystations Where, the channels'of communication from the aircraft are to be inserted into the same train of channel .pulses from. which lchannel `pulses are broadcast to the aircraft;

Fig. 11 is a graphical illustration used in .explaining the channel pulse broadcasting and inserting features of the embodiment shown in Fie. 10;

Fig. 12 is va block diagram of the aircraft equipment'that may be used inicoordination vwith the tower equipment illustrated' in Fig.. 10;

Fig. 13 is a block diagram Vof a terminal equipment fora further modification of the "system wherebyrchannelsrmay be selected for a given aircraftv and for broadcasting at a given relay station or block of relay stations;

Fig. 14 is a graphical illustration of channel pulses `according to the principles'of the embodilmentV shown in Fig. 13;

Fig. 15 is a block diagram of tower equipment that may be used for the selective broadcasting of channel pulses, the suppressing of channelpulses Jin the'train of channel pulsesv and the inserting Vof-channel pulses 4into the train;

'channel suppressor used in Fig. 15.

'Referring to Figs. 1 and 1A, two terminals IllA and I2 (west and east) areshown intercon- "n'ected by a chain ofrelay towers orstations I3,

I4, I5, etc. Each tower, particularly the towers between terminals, is provided with a plurality of antennas, antennas 2l, 2id. and 22, 22a. being directional as indicated while antennas 23V and 24 are preferably broad angle radiators, antennas 23 being used for broadcast radiation of channels to nearby aircraft and antennas 2li for reception 0f signals Vfrom aircraft. The directional antennas 2'I, 2Ia and l22, "22a are associated with east bound and west bound repeaters, respectively, for repeating channels of communication from tower to tower by narrow beam radio waves, successive frequencies F1, F2 and F3 being used for east bound transmission and frequencies F4, F5 and Fs for west bound transmission as indicated in Fig. 1.

In the rst embodiment of the invention, the antennas` et are employed for broadcasting on the same outgoing frequency (F1, F2 and Fa) as the transmitting antennas 22a. The receiving antennas 2li, however, are for wide angle .pickup oi' radio waves onfrequencyFi.

While the east bound channels are shown "in curve. 1.,`Fi'g. 1n, vfor example, as consisting of twelve "series `0`f very 'brief pulses, 'It Will beunder- Stood Vthat a greater.' or lesser, number of channels may be' used as desired', 'the [pulses 0f 'each Series being time modulated and 'so 'timed v'or phased from channelt chaln'elthatthe pulses 0f the twelve series interleave together "to form asin'g'le train ol. channel pulses. `Th`e separate pulses may be bf any desired width such. as one-tenth o'f a microsecond or one microsecond, yor larger, depending upon the band Width, numberofchannels 'andfpo'wer consumption permissible. 'The narrower the pulse the widerltheband width required, although alarger'nurnber of'channels then permissible. 'Ihe'widerthe pulse the greater is 'the average 'power consumption for a given peak power. It 'Will thus be understod'thatlfor a given 'pulse recurrence ratejper clianiie'LjSay a puls'eeVer'y'BO microseconds for example, more than 1GO channels Imay `vbe providedincludin'gfan allowance of 4as much as one-half afmicro'second displacement interval `per.. pulse for time .modulation.

The West bound .train of vchannel pulses utilizing frequencies F4, F5 or Fs, as 'the case may be, can'be similar .to that of the eastbound train of channel pulses, but preferably it includesin,.ad ,dit-ion tothe twelve channels of.communicationl a gap for insertion of at least six-inbranching channelsfrom aircraft. It will beunderstood. .of course, that many more channels may be selected thanV the twelve'given.herebyA way of examplefthe spacing Ybetween rpulsesoi adjacent. channelsefor a number larger than twelve being reducedaccordingly.

If fdesired, one :ofrtheA channels mayfbe usedI -for facsimile -transmission :of -maps, .'news, letc. For exampleppulses may be transmittedeas-facsimile signals, lthe pulses of -a selected". channel being used directly: asl the. facsimilebuild-.up characters.

It will also be understood that-whilerdisclose mycommunication system-eas faz pulse system to illustratethe principles of myfinvention,-'I donot lintend :to so `limit 'my v4invention. For example,

'not only different-.forms 4*of 'pulsing 1 systems may be employed, "but alsosAM. 'and jmo'dulated carrier wave systems might be employed;by using an appropriatev selection gofff-requencies' fa'nd/ or sub-carriers.

For the'presente'xanriple; -letf it' be-supposedithat first '-sixof f` the west'Y bund channels farlefrequiretl for two-way communication between the West terminal I0 and aircraft. The channels 'I to I2, both east bound and West bound, will then provide through communication between terminals, and may be referred to as east bound express channels and west bound express channels. respectively. The channels I to 6 for transmission to and from aircraft may be referred to as east bound branching channels and west bound branching channels, respectively. The No. I channel (Fig. 2) is preferably used as a synchronizing, monitoring and as an order channel, while channels 2 to S may be used for communicating purposes in the manner of party telephone lines as regards the aircraft in flight between terminals.

Referring to Fig. 2, the west terminal is shown to include a switchboard 30 containing jack connections for outgoing and incoming channels I-N and Ilz-Na, respectively. The switchboard is of known character provided for connecting telephone circuits or trunk lines to the proper channel jacks and also for connecting together Y incoming and outgoing channels under certain circumstances described hereinafter. Channel I includes a modulator 3l the input connection of which includes a lter 21 the function of which is described in connection with Fig. 6. The modulator 3| and also modulator 32 to the modulator for channel N may be of any desired type but are preferably of the character for producing pushpull time displacement modulation of pulses. In order to effect this time modulation, energy from a base wave source lill is applied over individual phase shifters l2- 41, etc., for channels 2 through N, respectively. Channel I need not have an individual phase shifter since the phase of the wave from source All) may be applied directly without phase shift to the modulator 3l. The phase Shifters for the other channels are adjusted so that the individual trains of pulses for the separate channels will be displaced in time to provide a single resultant train in the common output 5i) as indicated by the train of pulses of curve a, in Fig. 1A.

Channel I has been chosen as the synchronizing and monitoring channel as indicated by the connection of a source 5I of synchronizing signals to modulator 2l. The source 5l supplies energy to the modulator at a frequency f1 which imposes a given tone or time displacement on the pulses of channel l so as to distinguish channel I from all of the other outgoing channels. The synchronizing frequency f1 is distinguished from intelligence signals applied to the modulator by means of filter 21 which operates to exclude from the signals the synchronizing frequencies chosen as the synchronizing signal. Other synchronizing frequencies, as will be seen hereinafter, may be applied to other channel modulators for drop channel and other monitoring purposes.

All the trains of pulses produced by the different channel modulators (3| to N) are combined, as indicated by the output connection at 50, and applied to an R.F. translator 513 and radiated by transmitting antenna 2la.. The R.-F. translator in this instance will provide a carrier wave of frequency F1 as indicated in Fig. 1. This train of multiplex channel pulses is east bound, channel I operating as a synchronizing and monitoring channel for controlling the separation of channel pulses at the receiver whether in a plane or at the east terminal I2.

Before discussing the equipment and operation of the relay stations, let it be assumed insofar as the west base is now concerned that a west bound train of channel pulses la, 2a, 3a Na is being received on antenna 22, Fig. 2, and that the pulses thereof are detected into video form at receiver 30. The video pulses of the incoming channels are applied to a deblocking selector 6I of a synchronizing and monitoring unit S2. The video pulses passed by the selector 6I are applied to a demodulator 6 the output of which is applied to lters 65 and 66. Assuming that the pulses of channel la are modulated with a synchronizing frequency f2, filter 65 Will operate as an f2 blocking lter while filter GS Will operate to pass frequency f2 and exclude signal frequencies with which channel I may be moduated. The output of filter 66 is applied to a deblocking oscillator 61 which serves to produce timed pulses preferably of substantially rectangular form for application to a deblocking selector 6I.

Preferably, the deblccking oscillator 6'! is normally timed to provide a pulse timing slightly greater than the interval spacing of the pulses of any one channel. Consequently, when the pulses arrive at the deblocking selector 6I they are selected in rotation until such time as channel la -is received. Upon receipt of channel la, the demodulator 64 produces an output tone corresponding to frequency fz with which channel la is modulated. This synchronizing frequency is selected by filter S6 `and serves to synchronize the deblocking oscillator 5l which thereupon produces a deblocking pulse (see rectangular pulse 223, Fig. IA) which is timed according to the occurrence of the pulses of channel Ia. Thus, the progression of selection is halted and the selected channel (channel la) is continuously received thereafter by selector 6l. An understanding of the details of this particular selector arrangement is not necessary to a complete understanding of our invention and is therefore not set forth in detail herein although further explanation is given later in connection with Fig. '7. For a more complete understanding of this feature7 reference may be had to the copending application of D. D. Grieg, entitled Multiplex synchronizing system, Serial No. 514,998, led December 20, 1943 now Patent No. 2,418,116 issued April 1, 1947.

Energy from the debloclring oscillator 67 is also applied to the deblocking selectors of the other receiving channels, the energy being applied through suitable delay devices for proper timing. Referring to channel 2d, for example, the deblocking pulse produced by the deblocking oscillator 61 is applied to a delay device 'l0 and thence to deblocking selector I2 where it controls the selection of the pulses of channel 2a as received over line l l, the selector excludingall other channel pulses. This is illustrated by deblocking pulse 224 in Fig. 1A as described hereinafter in more detail in connection with Fig. 7. The channel pulses passed :by the selector l2 are applied to demodulator 14 whereby the time displacement of the video pulses is translated into an audio signal wave which in turn is applied over line 'I5 to the proper jack on the switchboard 3U, This selector control is the same for the additional channels 3a to Na and therefore need not be illustrated or described in detail.

The tower communicating equipment is shown as blocks and 8l in Figs. 1 and 3. Each repeater includes an R.F. detector, amplifier and R.-F. translator as indicated, for example, by blocks |50, I'EI and I52 in the illustration of repeater BI, Fig. 3. The block 80 comprises the repeater equip@ ment foreastzbounsdfchannels.the. repeaterbeing.- associated with input: antenna 211i and. output antenna-21u.. For purposesof broadcasting the branching channels, antenna 3v may be connected directly to the terminals of antenna2|c with proper impedance matching, ofcourse. This arrangement will broadcast all ofl the pulses .re-.' gardl'ess-of whether or not they are .branching or expressl channels. It may be desired to provide the equipment 80 with separate. transmittingy means whereby only the branchingv channels ,are broadcast or where selected onesofther branchN ingchannels are broadcast.. This selective feature, however, is the subject matter of a modification ofthe invention hereinafter described,

The. antennas 22my and 23 may be replaced by a'single wide angle directional antenna. That is to say, the transmitting antenna. would in such case have afwide. angle directional radiation like that indicated at 82 vin Fig. 3A, so that aircraft may. receive the signals when flying forward of theantenna. Radiation 83 is illustrative of -a narrow angle directional beam normally preferred between repeater stations. These antenna indications are, of course, diagrammatical since antenna arrays and structure for producing clifferent angle beamsare well known.

Referring now to Fig. 4, the channely pulses broadcast `from the antenna 23 may be detected on antenna 84 of an R.-F. detector 85 comprising apart of the aircraft communicating equipment. The .R.,-F. detector 85 is tunable, at least between frequenciesFi, F2 and Fs, whereby the operator can readily switch from one carrier to the next as'the aircraft moves from the .range of. one relay station into the range of the next. The output 36 of the detector 85 is applied to a synchronizing and monitoring unit 520, substantially identical to the form 62 illustrative in Fig. 2. The only difference is that the blocking filter 65a is adjusted to block frequencies f1 while filter 66a is adjusted to pass the frequencies f1. As described in connectionwith the unit S2 of 2, the deblocking oscillator 67 ties in with the synchronizing pulses of channel so that the earphones S13 receive `theintelligence signals with which channel l may be modulated. The deblocking pulses (rectangular pulses 223, Fig. 1A) of the deblocking oscillator 6l are applied over connection t' to delay device 9@ to provide delayed deblocking pulses (pulses 22s, Fig. 1A) having the proper timing for deblocling selector 92 for reception of the pulses of channel 2. The selector 92 thus passes the pulses of channel 2 to demodulator 94 whereupon the pulses are translated into amplitude modulated signal waves for reception at the usual reproducer.

Channels 3 to maybe banked for selective demodulation by an adjustable delay device lili. The devices 99 and lii preferably are of a known pulse translation type rather than the weighty network type. The adjustment of device ilii'at indication 3` infers that the device provides the necessary time delay to give the deblocking pulse the timing required for the pulses of channel 3.. The output of the device il is applied to a deblocking selector iZ to which the incoming video pulses are applied over connection |03. The selector thus passes, for the present adjustment of device Hill. the pulses of channel 3 which are demodulated at demodulator me, the output of which is applied to phones |05. By adjusting the delay device to indication il., the pulses of channel 4 will be selected and demdlllated. Likewise, adiustment7 to indications 5 and 6i will; cause thepulses: oil channels .5; and Beto-be dem'odulated,4 respectively.

For, communicating back iromthe aircraitto the multi-channel system, abase wavegenerator ||-|i1isvprovided for applying a .base wave to the. modulators I t2, I |.4 and, i i5.-A The wave to modu: laters- H4 and i limust .be shifted. relative to the Wave applied to modulator i ifi by phase VShifters |f|.3and ||.5 in :order to timethe output of the different modulators for interleaving of the pulse transmission. Inthe present form, the modulators are indicatedas ypulse modulators and the output of thegmoduletors, |2, liti and H6 is applied tof. pulse Shapers |22, ige and |26,respec tively.. The Shaper |22 is adjusted to produce a given pulserwidth W1, shaper |24 is adjusted torgive a. diierentpulse width W2 and Shaper |26.,is; made; adjustable to give a selection of pulse widths W3 to We. These different pulse widthswhichV may differby as small as one hundredthof aimicrosecond distinguish the pulses ofthe six channels la to tot. The common output i3.from the Shapers ifi?, |2i and |26 is appliedto an, R.-F. translator 53| through a suitable amplifier |'i.`|, to transmit the pulses on carrier. Fn over antenna |32. it will be noted that in Fig. .1i .the aircraft equipment is arranged forsirnultaneous communication over three channels. In some instances, no more than two channels are necessary. for simultaneous operation andin such cases` the additional equipment for channel 2 may be omitted and an additional adjustment provided on the delaydevice Mii) and Shaper |26 to include channel 2. This then would leave a-continuous indication for channel andselective communication over channels 2 to t. Channel will always be in operation so thatit not only operates to synchronize the reception of the equipment but also to provide an order channel over which the operator'at' aterminal station may order the plane tc communicate over a specified channel'. The order channel may also be used for supplying navigation data and traffic control instructions where the channel is not loaded with order requests. Should the route over which the aircraft travel be of ka character requiring that navigation data and instructions be given over a separate channel', then channel 2 as shown in 4.-, separate from the selective feature of devices iiii Aand' Shaper |26; willV be desirable.

Referring back to Fig. 3, the repeater block 8|' for the. west bound channels is shown to comprise an R.F. detector i513', amplifier |`5l and IRL-Ff. translator i5?" for repeating the train of channel pulses along the directive radiation path of the system together with associated equipment for receiving signals from anaircraft and inserting the signals in the form of pulses into appropriate intervals in the train of channelpulses relayed by units |53, |'5l'and 52. 1n order to effect the insertion synchronously into the train ofchannel pulses, a synchronizing pulse such as represented by channel la from east terminal l2 is selected by unit 62h. Pulse energy is fed in video form from the output of amplier |j5| to the synchronizing unit 62h. The unit 62bi's substantially the same as the unit 62 of Fig; 2 except the. filter 6'5 is omitted since there is no p .oint in. extracting the audio or other signals of channell. The deblocking pulse produced by the oscillator (il is appliedover connection |56 to a base wave generator |51' whereby' a base wave is producedin synchronism with the synchroni'zing pulses. of channel la.

Curve a of Fig. 5 illustrates the train of channel pulses relayed by the repeater station '81. The pulse la represents the synchronizing channel as well as the order channel to be used at the aircraft. Channels 2a through 6a represent the inbranching channels for communication from aircraft. The channel pulses 1a through Na represent express channels west bound from terminal |2 to terminal |0. A base wave |60 is produced by generator |51 in synchronism with the channel pulses Ia as shown by curve 5b. The wave |60 may therefore be used as the base wave for the modulator |62 for insertion of the aircraft order channel signals Ia. It will be understood, of course, that the order channel is not in continuous use but may serve a large number of aircraft in flight even though there may be some overlapping of conversations.

Since the channels transmitted by aircraft in flight are distinguished by a pulse width, the different channels may be segregated by means of pulse width selectors. The pulse width W1, for example, is segregated from the other pulse widths by selector |65 which may be cf any known pulse width discriminating circuit but preferably is of the character illustrated in detail in Fig. 8, the details of which are described hereinafter. The pulses of width W1 being thus segregated from the others may be demodulated to an audio wave by demodulator |56 and applied to the pulse generator and modulator |62 for insertion in the train of channel pulses at translator |52. This insertion is accomplished by output connection |63 and common input connection |64 to translator |52 in known manner. The other channels of communication from aircraft to relay station are likewise segregated by other pulse width selectors such as selector for pulse Width We. Since the position for channel 6a as well as channels 2a to 5a are different from channel la, phase Shifters as indicated at |16 are required for proper phasing of the base wave provided for the modulators of such channels.

The phase shift for the base wave for channels 2a and 3a. is represented by the waves |18 and |19, curve 5b. The pulses inserted by the waves |60, |18 and |10 are indicated by curve 5c. 5d represents the channel pulses as transmitted over antenna 22a. from the repeater 8|. It will be noted that at this point in the chain of repeaters that branching channels la, 2a., 3a and 5a are now occupied. Channels la and 6a are unoccupied and may be assigned to other aircraft for communication with the base terminal or other point in the system.

From the foregoing description, it will be clear that outbranching channels to 6, Fig. lA, are radiated at a broad angle at the relay stations or at certain of the stations since, obviously, it will not be necessary that the outbranching channels be broadcast every or 30 miles along the line of communication. The relay stations along the lines at which broadcasting of the outbranching channels are desirable will depend, to a large extent, upon the terrain and the power at which such broadcasting is to be made. These outbranching channels are selectively receivable by the aircraft equipment and return channels of communication and are transmitted from the aircraft to the relay stations for insertion into the train of west bound channel pulses. The detection of return channels from the aircraft and the synchronizing of the insertion at the relay station avoid interference overlapping of channel pulses by inbranching channel pulses at the base terminal. This is true regardless of the fact that two or more relay stations may receive the same inbranching channels from a given aircraft. This double or triple reception of the same signals by two or more relay stations means that the overlapping relation to pulses for a given signal have only a very small difference in time, and such difference is so slight as to be negligible insofar as the time modulation of the pulses is concerned.

In order more clearly to disclose the structural examples of a pulse generator and modulator suitable for use in the transmitter circuits of Figs. 2, 3 and 4, reference may be had to the circuit diagram of Fig. 6. Signal energy for channel Fig. 2, for example, is applied over a lter 21 to modulating circuit 3|, the output of which is coupled to the outgoing line 50. The filter 21 in the grid circuit to coupling tube 200 is tuned to exclude from the audio input to the modulator the/synchronizing frequencies selected at source 5|. Simultaneously, synchronizing modulations from source 5| are applied to a second coupling tube 20|. The outputs of these tubes are combined in line 202 producing a resultant mixing of the signals and synchronizing frequencies. This resultant mixed wave is applied over separate transformer windings 204 and 205 to a mixing transformer arrangement 203. Condenser 2|6 serves to remove the direct current from the primary winding. Simultaneously, energy from base Wave generator 40 is applied to mixing transformer 203 by means of coil 206. Secondary coils 201 and 208 are also coupled to transformer 203 to extract from there the mixed signals. Coils 201 and 208 are coupled to a pair of triode amplifiers 200 and 2|0 which are offset biased by batteries 2|3 and 2| 3a, for operation in the manner of a full-wave rectifier. The base wave 2| I, Fig. 6, of generator 40 preferably has a frequency corresponding to the desired cadence frequency T of the pulses to be produced.

Since tubes 209 and 2|0 operate in eifect as a full-Wave rectifier relative to the offset bias 2| la caused by potentials 2|3 and 2|3a, an output Wave of the form shown at 2|2 will be produced in the absence of any input signals. This wave is applied to a pulse shaper 2M, Fig. 6, which clips and shapes the cusps of wave 2| 2 to produce a plurality of pulses offset as indicated by the solid line pulses of pulse train 2|5, the pulses having a cadence frequency T as indicated. Upon application of energy from sources 5| and/or over channel connection 2|1, the offset bias of the push-pull circuit of tubes 209, 2|0 is varied producing pulses having substantially the same cadence frequency as before but displaced in .push-pull between the maximum limits indicated by the broken line pulses 2 i8 of pulse train 2 5. Thus, as the signals are applied to modulate the base Wave, a time displacement of the pulses in the output of pulse Shaper 2M is provided. While Fig. 6 discloses a modulator circuit for block 3l of Fig. 2, it is clear that the same type of modulator may be used in all of the different pulse generator and modulator circuits of the system. In channels in which no synchronizing signals are to be applied, synchronizing signal source 5| may be omitted. It is important, however, that the base Wave generator be controlled or at least synchronized to the synchronizing signals of source 5| in order to provide for proper segregation or dropping of channel pulses at receivers in the system. For a further description of this type of modulator, reference may-be made to the copend-ing applica-tion of E;,Labin and D. D. Grieg,

Serial No. 455,897, filed August 24, i942, now Patent No. 2,416,329, issued February 25, 1947. It will be understood, of course, that other` forms of modulators may be used, also that the modulator if .of the push-pully cusper type may be symmetrically biasedY as hereinbefore mentioned.

A typical demodulator circuitior time modulated pulses is shown in Fig. '7. This circuit corresponds to the combination deblocking selector andv demodulatorv arrangement 52' hereinbefore referredY to as a synchronizing and monitoring unit, Figs. 2, 3' and 4. It should be distinctly understood, however,I that the demodulator V54' of this circuit'. may beAused for demodulation in the other parts of thesystem.

According .torthe arrangement of Fig. '7,r the train of pulses from line 53 inV Fig. 2, for, exan.- ple, is fed over line 53a to mixer tube 22u of the deblocking selector 5l. At the same time, deblocking pulses from deblocker oscillator 51 are fed to another separate grid of tube 200 over line 222. These deblocking pulses serve to produce, inV conjunction with the incoming. pulses timed to add thereto, anoutput series of pulses from tube 220. Assume,v for example, that channel pulses of curve a, Fig. 1A, are applied to the selector' 6l, the deblocking oscillator willl operate to produce a plurality of rectangular pulses 223, curve btimed in spaced relationaccording tothe pulses of channel IY. Tube 225 is also biased to serve as a clipper so that, only the boosted pulses oi channel l appear in the output thereof. A D. C. restorer rectifier tube 2,3!) may be provided acrossl the input from deblocking oscillator 51 to assure that the incoming deblocking pulses are of a proper level to workwith the clipper circuit of.

tube 225' to leave only the desired output pulses above clipping level- 228, curve b, Fig. 1A.

- The outputpulses from tube 220,' have the same cadence frequency T asv the original modulated pulses 215 from modulator 3|, Fig. 6. The pulses are applied to one-of the grids of a demodulator tube 240 and cause tuned circuit 24d connected to another grid of this tube to oscillate at a desired frequency producing in the output of tubev 245. aV combined wave in the form of a combination of the wave generated in 241 andthe incoming pulses. Circuit 24 is preferably tuned4 to some harmonic of the cadence frequency of the input pulses so thatY as the pulse displacement is varied, due to the modulation signals, the output pulses will bel raised to dierentleyels depending upon their time displacement.V Accordingly,v in the output of vtube 24e will appear a modulation envelope of pulses carrying signal modulations' thereon. For a further understanding of the principles of 'this type-demodulator reference may be had to the copending application of D. D; Grieg, SeriaLNo. 459,959,7i'lled September 28, 1942', now Patent No. 2,416,306v issued February 25, 1947 A- low-pass lter 245 is Vprovided to remove from this demodulated signal envelope thepulses of higher frequencies that define the signal en- Vvelope. `The output synchronizing signal' is passed over lter 66 tuned to pass only the synchronizing. frequency such as f2Fig. 2, which, in turn, serves to synchronize the operation of' debloclring oscillator 61. The remaining demodulated frequencies ofthe input pulses are passed to a reproducer by filter65 tunedv to exclude the synchronizing signal frequency. It should be distinctlyunderstood that in cases where the deblocking oscillator is not controlled directly byV trolled by adjustment of resistor 211.

Y 112 the channel to beselected, the deblocking pulses applied to tube 225 may be produced from a synchronizing signal of aV different channe] and retarded asl by delay device 15, Fig. 2, `to have aY timing according to the pulses of the channel to be received.

While the pulse width selectors |55, H15 of Fig.` 3-may comprise any circuit arrangement capableV ofv segregating a pulse of a given width from pulses of greater and/or lesseifwidths, I have Y shown in Fig. 8, for purposes of illustration, a suitable circuit which, depending upon 'selectionof circuit constants, iscapable of distinguishing between two pulse widths differing by as little as one hundredth of a microsecond. Such line distinction, however, is notI necessary except where a very large number of diierent pulse widths areI required between close limits.

The circuit of Fig. 8V preferably includes a limit clipping stage 250 as an input coupler which limits all input pulses to substantially the same amplitude. Should the input pulses be of a positive polarity as indicated by the pulses of curveconnected to the opposite side 253 of the'tuna'bleV circuit. rIheV side 263 is also connected to a source of anode potential 254. The pulse energy, curve 9b, from the anode connection 254 is applied to the grid 255 of the tube 265 so as to block the conduction between the cathode 26| and the anode 262 while pulse energy is applied to the circuit 255,'. The undulations produced in the circuit 255 in response to pulse energyl over anode connection 254 are taken off'through a connection 211]A for application to a threshold clipping amplifier'stage 215. The bias on the grid 215V is con- In the output 218 of stage 215 is a pulse width Shaper 28,0 the operation of which is hereinafter described.

Assume for purposes of illustration, that the widths of the pulses of. curves 9a and 9b correspond, respectively, to channels I, 2, 3, l and 5 as indicated by the width'reference characters W1, W2, Ws, W4 and W5. Assume also that the circuit 255 is tuned for selection of lpulse width W3. Curve 9c represents the output of the circuit 2'55 when the circuit 255 is tuned for selection. of pulse width W3, illustrating the diilerent output undulations for the different pulse widths of curve 9a. When theleading edge 28| of the pulse W3Y is applied at negative polarity as indicated by curve 9b to the'circuit 255, an initial undulation 282 is produced which is normally followed by undulations 283, 284 and so on in theiform ofY a damped wave. When th-e circuit 255 is tuned to a frequency the period of which is exactly twice the width W3, the trailing edge 285 occurs where the initiated oscillatory energy crosses the Zero axis Vfrom undulation'282'to undulationV 283. Since the, trailing edge 286 shock excites the circuit inthe same direction at this point, the undulation 281 produced thereby in the circuit 255 adds algebraically to the undulation 283 to produce undulation 290; The next succeeding pairs of undulations produced by the leading and trailing edges of pulse Width W3 would normally tend to produce a negative '13 undulation 29| which would continue as a' damped wave as indicated at 292. The damping tube 250, however, eliminates the trailing oscillations 292 so that they do not interfere with the undulations produced by subsequent pulses applied to the circuit 255.

A pulse width less than pulse width W3 such, for example, as pulse widths W1 and W2, will not produce maximum undulations as great as the undulation 250 for the tuning adjustment corresponding to pulse width W3. This is illustrated by the undulations 253 and 294 produced in response to the pulse widths W1 and W2, respectively. The reason for this is readily apparent because the shock excitations produced by the leading and trailing edges of the pulses of lesser width than W3 are in part opposed to each other as indicated by the broken lines associated with the undulations 233 and 224. The undulations 285 and 296 produced in response to the greater pulse widths W4 and W5 are likewise smaller than the undulation 290 since here again the oscillations produced in response to the leading and trailing edges of the greater pulse widths are in part opposed to each other so that the algebriac summation thereof is less than in the case of the undulations produced in response to pulse width W3.

The threshold clipping stage 2'l5 is adjusted to clip at a level 29'! thereby obtaining and amplifying the crest portion 20041, of the undulation 290 as indicated by curve 9d. The pulse Shaper 280 is preferably of the character adapted to differentiate the pulse 29M producing the pulse shape 29013 of curve 3e. The shaper also includes a clipper stage for clipping the positive pulse portion of pulse shape 25013 at level 293 thereby producing a narrow width pulse 230e synchronized in time to the pulse width W3, Thus, any time modulation applied to the pulses of width W3 will carry through to the output pulse 299e which may be demodulat'ed to an audio wave by the demodulator M. of Fig. '7. It will also be readily apparent that by adjusting the tuning of circuit 255 to another frequency the period of which is twice the duration of any one of the other pulse widths of curve 9a, that a corresponding output pulse will be produced representing the pulses of the pulse width selected,

A second embodiment of the invention is disclosed in Figs. l0, 1l and l2. This embodiment provides for broadcasting the outbranching channels only at the relay stations, the express channels being transmitted on the narrow beam from relay station to relay station, and for inserting inbranching channels on the same chain of channel pulses containing the above-mentioned outbranching channels.

In Fig. l0, the relay repeater 300 for east bound channels is shown to include an R.F. detector 30I wherein the pulses of carrier frequency F1 are translated into video form and applied to deblocking pulse producer 62h, the elements of which are identical to the corresponding elements of the synchronizing and monitoring unit 62 of Fig. 2 except that here the audio of channel I is not segregated by a second iilter. The deblocking pulse produced by the deblocking oscillator 61 is applied to a second deblocking oscillator 304 adjusted to produce a deblocking pulse 355 of a duration suicient to deblock all of the outbranching channels I to S (see curves lia to lid of Fig. l1).

If desired, the oscilla-tor 304 may be adjusted to provide deblocking pulses of lesser width as indicated by the broken lines 303, thereby selecting the pulses of channels I, 2 and 3 for broadcasting at such relay station. This selection may be diiferent for different relay stations or blocks of such stations thereby limiting the broadcasting at certain relay stations to certain selected outbranching channels.

The deblocking pulses produced by oscillator 304 are applied to a deblocking selector 306 to which the video pulses of the detector 30| are applied over connection 301. The deblocking pulses 305 thus condition the selector 306 to pass the outbranching channel pulses I to 6 for translation to R..F. frequency Fa by translator 3I0 for radiation over wide angle antenna SI2.

It will be understood, of course, that the outbranching as well as the express channels are applied from the detector 30| to the amplier and R.F. translator 3I5 and radiated over directive antenna 2 la, in the manner heretofore described. Thus, the tower equipment is capable of selecting the outbranching channels at each tower and for wide angle radiation distinct from the directional repeating function of the repeater equipment.

Referring now to Fig. 12, the aircraft communieating equipment is shown to comprise an R.F. detector 320 whereby the outbranching channel pulses transmitted by the tower antenna 3I2, Fig. l0, are received and translated into video form. The video pulses are applied to a synchronizing unit 62 identical to that used in Fig. 2 whereby the synchronizing signal of channel I synchronizes the receiver equipment to the timing relationship of the outbranching channel pulses. As hereinbefore described the deblocking oscillator 61 produces a deblocking pulse timed to each pulse of channel I so that the selector 5I will pass the pulses of channel i which are demodulated at 64 and the audio signals thereof passed by lter 65 to earphones 32I. The deblocking pulses produced by the oscillator Si are applied to an adjustable delay device 322 over connection 323 where the deblocking pulses are delayed a selective amount depending on the adjustment for deblocking a selected one of the outbranching channels 2, 3, A, 5 and 6. For minimum weight the device 323 is preferably of the pulse translation type rather than the network type. As shown, the device 322 is adjusted to provide a delay suicient to time the deblocking pulses for the pulses of channel 2 so that by application of the delayed deblocking pulses to selector 325, the selector will pass only pulses of channel 2. The channel 2 pulses are then demodulated by demodulator 326 and the output thereof applied to earphones 321.

It will be readily apparent from the foregoing description that by changing the adjustment of the delay device 322 reception may be had over any one of the channels 2 to 6. In normal operation, the operator at the base terminal will designate over an order channel, in this case channel I, which channel the operator at the plane is to receive a message and also which channel he may use in replying to same. Should the inbranching channel designated be channel 5, the delay device 322 would be changed to the adjustment No. 5 and the phone 32T would then receive messages coming over channel 5. This adjustment of the device 322 would preferably be by some simple step by step system.

The transmitting equipment 330 for the aircraft is shown in this embodiment, by way of a further example, to comprise A.M. modulation of carrier waves, a different carrier wave being selected for each channel. Inbranching channel I, or in other words the order channel from aircraft to terminal, is transmitted by an R.F. translator 332 coupled'to an antenna. 333, to which voicev or other intelligence signal is applied over microphone 336i and audio amplifier 335. The carrier wave is supplied by a bank of oscillators 340. @utput connection 35H provides a carrier frequency ,fa for inbranching channel l. 'Ihis frequency is applied to the translator 332 and thus serves as the carrier for the intelligence received at microphone 333. The output connections 342V through connection 345 provide carrier frequencies fb, fa, fd, je and fr, respectively, to frequency selector 35u where, by a movable contact 357|Y or other selective arrangement, selective connection with lone or the other of the output connections 342 to 3&6 may be made for application of the carrier energy thereof to R.F; translator 354. It will thus be clear that by proper selection of the carrier, a reply to conversation received over any one of the outbranching channels. 2 to 6 may be made over any designated inbranching channel frequency.

Turning back to Fig. 10, the transmission of the inbranching channel carriers from the aircraft equipment of Fig. l2 is receivedA over broad band antenna 353v which is coupled to R.F. detectors 33! through 335. The detector 36| isv tuned for narrow band reception of carrier wave ja, the audio output of which is applied to an inbranching channel modulator 3l! for channel Ib. Ther other inbranching carriers fb, fc, etc., are likewise detected to audio and applied to modulators 312, T3, etc.

The employment of A.M. modulationrfor the inbranching channels raises the question of fading due to variation of the distance between the plane and receiving towers. This difficulty, Vof course, may be compensated for by using A.V.C. circuit.

In order to insert the channel pulsesof. modulators 3l! to 313 in a given gap in the train of channel pulses, curve l la, of Fig. 11, it is important to synchronize the pulse generating function of the modulators. This is accomplished by applying the deblocking pulse of oscillator 61, Fig. 10, over connection 3H to a base wave generator 318, the output of which is applied to a bank of phase shifters 3B! to 386. The base wave produced by generator 318 may thus be shifted in phase for application to each of the different modulators 3H to 313. As hereinbefore described in connection with Fig. 6, the timing of, the pulses produced by the different modulators is controlled so that the inbranching pulses will interleave together into the gap shown in Fig. 11a'.

It will be understood, of course, that a plane can according to the illustration ofthe transmitter 33B in Fig. 12 transmit two channels at a time, one an order channel'and the other a Vselected communication channel. According to the embodiment illustrated, five planes can transmit intelligence simultaneously over channels separate from the order channel to the base terminal or to other points in the system. Curves i le l if and i lg represent three busy inbranching channels from planes E, F and G, the second, third and fourth inbranching channels being used, respectively. These inbranching channel pulses may be received by the same equipment on a given tower or by the equipment from a dif-V ferent tower, the synchronization thereof being controlled by the order channel l from the base terminal. The inbranching order channel is shown to be occupied by the transmitter equip ment on plane H by the channel pulse shown in curve Hh. The fthlv and sixth inbranching channels are shown to be unoccupied. Should two or more planes try to occupy the vinbranching order channel simultaneously, some confusion may result but this may be overcome by the yoperator at the base terminal designating which plane is to use the order channel.

Since the inbranching channel pulses are inserted inthe same east bound train of channel pulses having the outbranching channel pulses, express' channels from the east to west terminals will be used, in this embodiment, for the` inbranching order channel and other inbranching channels that must be trunked to or through the west terminal. This trunking of the channels from east to west will be obvious to those skilled, in the communication art and therefore need not be illustrated for a clear understanding .of the invention.

Figs. 13 to 18 illustrate a further embodiment;` of the invention whereinthe operator at the west terminal is capable of transmitting a selected channel to a selectedv relay tower for transmission from such tower independent of other Vrelay towers for aircraft reception, and whereby inbranching signals from the aircraft may be received by such tower` and' transl-ated into signal modulated pulses to replace the pulses of the channel broadcast at such tower.

Fig. 13 shows the west terminal equipment which, as inthe case of the terminal equipment of Fig. 2, may comprise a switchboard 400- from which signals for the diiferent channels are applied to the respective channel pulse generating and shaping equipment for transmission by common pulse amplifier 385 and R.F. translator 409. The signals for channel l are applied toa pulse generator and modulator 432 which may be ofv the character shown in detail in Fig. 6. The base Wave for the modulator isproduced by generator 465. A pulse shaper 405 provides the pulses of channel l with a given width To which operates as the order channel for the system and is broadcast by each of the relay towers. The pulse width for channel l is indicated by curve lila, Fig. 14, in comparison with a few other relativepulse widths that may be used for channels 2 to 6.

The pulses of channel 2 are generated by pulse` generator-and modulator li l 2, the base wave being supplied thereto from generator 435 through phase shifter 4H) to provide the pulses generated with the properv timing with respect to the pulses of channel l'. The pulses for channel 3 through channel-N are likewise produced at diiferent tim,- ings sothat they all interleave in the manner illustrated in curve Ida. The pulse shaper and width" selector 41'5 is adjustable to shape the pulses of channelV 2 for different widths. The other outbranching channels 3, 4, 5 and 6 are likewise provided with pulse Shaper and width selectors similar to selector M5. This width selection determines the tower relay station at which the pulses of the channels, 2, 3, 4, 5 or B are to be segregated from the pulses of other channels and broadcast to aircraft in the vicinity of such Instead of just one tower a given block Y craft reception at tower T1 or a given block of towers known as tower block T1. Curve I4a further shows channel 3 of a pulse width for broadcasting at tower Te, channel 4 for tower T3 and channel 5 for tower T10. Channel 5, however, is not shown in use, therefore no pulse is shown in the time position of channel 6. If a channel is not in use the base wave to the modulator of the channel may be discontinued by opening switch 4|6 (see channel 2, Fig. 13). Channels 1 to N are shown to be occupied by narrow width pulses narrower than the narrowest outbranching channel. This difference of the express channel pulse widths from the outbranching channel pulse widths insures that the express channels Will not be broadcast, suppressed or otherwise interfered with at the relay stations. It should be understood that the pulse Widths shown are relative and are exaggerated for clarity of illustration. Under actual practice the width 5, as hereinbefore stated, may differ by only a fraction of a microsecond,

Fig. 15 shows the relay equipment for tower T1. The train of R.F. pulses is detected by detector 420 and the video output thereof is applied to a channel suppressor 42 thence to a coupling amplier 422 and finally to an R.F. translator 423 whereby the pulses are applied to a carrier for transmission to the next relay station in the manner hereinbefore described. The video pulse output of detector 420 is applied over connection 425 toapulse width selector 426. This selector may be of the character described in connection with Figs. 8 and 9 whereby a particular pulse width To, for example, may be segregated from the other pulse widths present. A pulse width shaper 421 may be provided at the output of selector 426 to reshape the pulse to the width To or to any other pulse width desired. The output of the shaper 421 may then be applied to coupler 428 for application to R.F. translator 430 for transmission over wide angle antenna 432.

A second pulse width selector 431 is provided to which the video pulses of detector 420 are applied.

This pulse selector is adjusted for pulse width T1.:

The output of the selector 431 is applied over connection 438 to coupler 428 for broadcasting along with the channel pulses of width To. These pulses from selector 431, however, need not be reshaped to the initial width although they may if de-A sired. Before completing the description of Fig. 15, reference is made to Fig. 16 which illustrates the receiver equipment for aircraft. The receiver includes an R.F. detector 44S which translates the signal pulses to video for application to a pulse width selector 442. The selector 442 is adjusted for pulse width To so as to select only the order channel which is demodulated to audio at 443 and then reproduced at earphones 444. The selector reception of the other outbranching channels 2 to is controlled by an adjustable delay device 458 preferably of the tube circuit type to which the pulse output of selector 442 is applied. The device 451! is adjustable to five different positions one for each of the channels 2 to 5 for providing the desired delay to the pulses from selector 442 so Vthat they may be used as deblocking pulses for the deblocking selector 452. A pulse width shaper 445 is included to provide a proper pulse width sucient to deblock the selector 452 for passage of pulses of channel 2 or which other channel that may be initially transmitted of Width T1. n

When an order comes through from the west terminal to the aircraft over channel I to the earphones 444, Fig. 16, for separate communication over a given one of the channels 2 to 5, the adjustment of the device 45t to the proper delay position will cause the selector 452 to pass the pulses of the proper channel according to the time differential thereof with respect to channel The audio with which the channel pulses are modulated is then demodulated by demodulator 453 for reception over earphones 454.

For transmission from the aircraft to the tower relay station, the same transmitter equipment 333 disclosed in Fig. 12 may be used.

The receiver equipment at the tower, Fig. 15, includes broad band antenna 460 and a pair of R.F. detectors 451 and 452 with which the antenna is coupled. The frequency of the carriers may be in the same order described in connection with transmitter 33E) in Fig. 12. The carrier fa, for example, will correspond to the inbranching order channel as hereinbefore described and may be received by the rst detector 46|. The `additional carrier waves, however, are so selected (one per tower) that only one of them such as fb is receivable by the detector 462. Thus, the carrier fb corresponds to tower T1 and will be received only by such tower. These carrier frequencies, however, need not be multiplied to any greater number than illustrated in Fig. 12 since they may be repeated along the chain of relay tower stations, the carrier fb, for example, being the c-arrier receivable by the first, sixth, eleventh and sixteenth, etc., towers.

The inbranching channel signals being detected to audio at the detectors 46| and 482 are thus applied to the pulse generator and modulator units 41| and 412, respectively. The channel pulses produced by the units 41 and 412 may be inserted in gaps in the pulse train or the channel pulses broadcast by the translator 430 may be suppressed and their position in the train of pulses taken by the channel pulses produced by the modulators 41| and 412. For purposes of illustration, the outbranching channel pulse of width T1 is suppressed at tower T1 and the gap left thereby filled by the channel pulse produced by modulator 412. The inbranching order channel from the aircraft, the pulses of which are produced by modulator 41|, may be applied to the same position as the outbranching order channel or it may be timed for insertion at a different channel position such as channel 8, for example.

Referring particularly to Figs. 14, 15 and 17, the suppression of outbranching channel 2 of width T1 will now be described. The video output pulse of selector 431, Fig. 15, may be employed for this purpose by pulse widening, reversing in polarity and controlling the timing of the widened pulses for blocking the pulses of channel 2. For example, however, the blocking pulses are produced from a base wave generator 414 controlled by the pulse output of selector 426, although, if desired, it may be controlled by' the output of selector 431. The base wave is shifted for proper timing by phase shifter 415 and applied over line 415 to blocking pulse generator 411. The generator 411 may be of any suitable character and, for example, may be of the type shown in Fig. 6, the output pulses being shaped andreversed in polarity by clipper and amplifying circuits of known character. The blocking wave 485, produced by generator 411, is indicated in Fig. 17 and curve !4b,'Fig. 14.

The suppressor circuit 42|, Fig. 1'1, comprises a pair of vacuum tubes 48|, 482. Into tube 48| 

